An organic light emitting diode (OLED) is a self-emissive device having a wide viewing angle, an excellent contrast ratio, a fast response time, a high brightness, a high drive voltage and a high response speed, and capable of multi-colorization.
A general OLED may include a transparent substrate, an anode, a cathode, and an organic layer interposed between the anode and the cathode. The organic layer may include a hole injection layer, a hole transport layer, an emitting layer, an electron transport layer, and an electron injection layer. Holes injected from the anode are transferred to the emitting layer via the hole transport layer, and electrons injected from the cathode are transferred to an emitting layer via the electron transport layer. The carriers such as the holes and the electrons are recombined in the emitting layer region, thereby generating excitons, which are transformed to a ground state from an excited state, resulting in a generation of light.
In an early stage of an OLED research, a small molecule OLED having a simple device structure and manufactured by depositing a low molecule material through a vacuum process had been mainly studied, and in the 2000s, flexible organic electronics could be realized, and research for realizing polymer light-emitting diodes (PLED) by a solution process which is less expensive and enables mass-production such as spin-coating, ink jet printing or roll-to-roll (R2R) coating are actively progressing.
To realize OLEDs or PLEDs having high efficiency and a long lifespan, research on multi-layered devices is mainly reported. However, as several layers are applied, a production cost is increased, and thus in consideration of commercialization, research for realizing an OLED device simplified without an additional charge transport layer is a very important issue. Also, in addition to research of realizing a PLED which is simple, stable in the air, and manufactured by a solution process, application of a structure of a device not requiring encapsulation and a printing method to a device will become a meaningful research advancing application of an R2R process for mass-production. As one method for realizing this, development of an inverted PLED using an oxide semiconductor as an electron transport layer and forming an electrode stable in the air on the uppermost portion became important.
At this stage of increasing attention to new regeneration energy around the world, organic photovoltaic cells (OPVs) having probability as future energy and various advantages are receiving attention. The OPVs can be manufactured in a thin film at a low price, compared to inorganic photovoltaic cells using silicon, and may be applied to various future flexible devices in various ways.
A conventional OPV may include an anode, a hole injection layer, a light active layer and a cathode. The light irradiated onto the OPVs may be separated into electrons and holes in the light active layer. The holes may be extracted through the anode via the hole injection layer, and the electrons may be extracted through the cathode.
To solve issues of high efficiency, a long lifespan and a simple device structure, research on inverted OPVs using metal oxides such as TiO2, ZrO2, and ZnO is on the rise as the most representative solutions which are stable in the air and can be applied to an R2R process.
In the inverted device, in contrast with a structure of a general OLED or OPV device in which holes are extracted through a transparent electrode such as an indium tin oxide (ITO), electrons are extracted through the transparent electrode (e.g., ITO or FTO) to serve as a cathode, and anodes generally use a metal such as Au or Ag.
Due to such a structure, the inverted device may not use a highly-reactive electron injection electrode (that is, a cathode) used in the general OLED or OPV device, a metal such as Ca, Ba or Li, but may use materials having no reactivity to an air or moisture because both of the anode and cathode have high work functions. Although organic materials may be used as an electron injection layer of an inverted PLED or OPV device, particularly, since a metal oxide has high transparency in a visible region and high charge transport capability, and is stable in the air, there is various research for applying such a metal oxide formed in a solution process to the progressing inverted device.
However, when a conventional oxide electrode such as ITO or FTO, which is disposed on a glass substrate, is bent so as to cause a crack in a thin film, it cannot serve as an electrode any more. In addition, although the conventional glass substrate is used since a substrate should also withstand a process of depositing a metal oxide performed in a high temperature process at 200° C. or more, the glass substrate is not flexible. Accordingly, it is necessary to use a substrate and an electrode, which has excellent mechanical strength to be bendable and can endure a high temperature process of 200° C. or more. As such a substrate, a metal foil may be used, but the prior art disclosed that when the metal foil is used, a device is realized by further depositing an electrode after forming an insulating planarization layer, or a device having a complicated structure is realized by further depositing a metal serving as a planarization layer and a reflective conductive layer on the metal foil.